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BIOLOGY
CLASS: SS 2
TOPIC: POLLINATION IN PLANTS
CONTENT:
- Types of pollination.
- Features of self and cross pollinated flowers
- Features of wind and insect pollinated
- Agents of pollination
SUB-TOPIC 1: TYPES OF POLLINATION
Pollination is the process by which pollen grains from an anther of a flower are transferred to the stigma of the flower or another flower of the same species. In most species of fowering plant, external agent brings about the pollination. Flowers have evolved special structured and mechanisms to ensure successful pollination. The proce enables fertilisation and sexual reproduction to occur.
There are two types of pollination:
- Self pollination
- Cross pollination.
Self-pollination: this is the process by which mature pollen grains are transferred from the anthers of a flower to the stigma of the same flower (autogamy) or other flowers on the same plant (cleistogamy). It is common in short-lived annual species. This process has a high successful rate. Self-pollination brings the male and female gametes of the same plant together. The offspring show very little genetic variation.
Cross-pollination: this is the transfer of mature pollen grains from the anther of a flower to the stigma of a different flower of the same or closely related species. This process is risky and wasteful as most pollen grains fail to reach receptive stigma. Cross pollination brings the male and female gametes of two different parent plants together. There are great genetic variations among the offspring which tend to be healthy and well adapted.
EVALUATION:
- What is pollination?
- Describe the process of self pollination.
- Mention two advantages of cross pollination.
SUB-TOPIC 2: FEATURES OF SELF AND CROSS-POLLINATION
Self-pollination can occur only in bisexual flowers and in unisexual flowers of monoecious plants. The following are the adaptive features that favour self pollination:
- Cleistogamy: this occurs among closed flowers. The ripe pollen grains are deposited on the stigma which ripens later.
- Homogamy: the carpels and stamens in the bisexual flowers mature at the same time. This homogamous condition promotes self-pollination through one of these ways:
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- Pollination agents readily carry pollen grains from mature anthers to the receptive stigma.
- A slight wind may blow ripened pollen grains from a mature anther onto any receptive stigma.
- Mature stigma pushing out of the corolla tube may brush against the anther and collect pollen grains on the longest filaments.
- Filaments longer than the sigma may recoil and touch the mature stigmas.
- Styles longer than the filaments may bend to make the stigma touch the anther.
Evolution has favoured the development of plants and flowers that promoted cross-pollination. Many flowers, therefore, exhibit various features that enhance cross-pollination and hinder self-pollination.
These features include the following:
- Xenogamy: this is the transfer of pollen grains of a flower to the stigma of another flower situated on different plants of the same species.
- Hydrisation: this is the transfer of pollen grains of a flower of one species to the stigma of a flower of a different species.
- Unisexuality: this occurs when female and male flowers are borne on separate plants. (Dioecious plants) of pawpaw.
- Dichogamy: this is a condition in which the male and female reproductive organs of a flower mature of different times. Self-pollination is usually impossible in this situation.
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